The transgenic Mouse/ES Cell Shared Resource consists of both the Microinjection and ES Cell Laboratories. Together, these two components provide seven different services that facilitate the generation, maintenance, or storage of transgenic and gene knock- out mice. Over the past three and a half years this resource has served 35 different Cancer Center investigators (of a total of 50 different investigators). The Microinjection Laboratory has generated over 273 transgenic founder mice (made via DNA microinjections) and 508 chimeric mice (made via ES cell microinjections). The ES Cell laboratory, which began operation in July, 1995, has assisted in the generation of at least nine different gene-targeted mice. Four other new services have been developed and are now provided to investigators. These include rederivation of established lines, assisted reproduction, embryo cyropreservation, and rental of a microinjection apparatus for investigators who want to perform their own microinjection experiments. This resource, which has been successful in meeting requests for its services, is expected to continue to provide both existing and new services as the demand for germline-altered mice in biomedical investigation continues to grow.